How much cardio exercise should i do a day/week to prevent cardiovascular disease?

Depends: Prevention is an important modifier in your question. Prevention must be balanced against innate risks you may have. That being said, movement of almost any type on a daily basis is best, but "cardio" activity of 30-40 minutes 3-4 times per week may suffice to maintain conditioning. That's a big leap, though, to "prevention". Need to be vigilant against overhyping prevention.